What is a road novel? The answer is not straightforward. Does it actually have to physically involve a road? Is it a purely American form of literature, conjured by the vast networks of freeway and the dominant car culture? Is it an essentially 20th-century genre? And does it even have to be a novel?

My answers, for what they're worth, are as follows. No, it needn't involve a road, but probably will. Yes, it is pretty much an American form. Yes, it is essentially 20th-century, with exceptions. And yes, it does have to be a novel (which disqualifies The Electric Kool-Aid Acid Test). By this definition, a road novel would still include, say, The Grapes of Wrath, which nevertheless somehow doesn't quite fit - mainly because it is a novel about desperation and escape rather than exploration and adventure, which to my mind are the quintessence of the road novel.

It's not simple. In my choice of three great road novels, the purest is (arguably) the father of whole genre, On the Road by Jack Kerouac. Purest, but not pure, because it is heavily autobiographical and to my mind only just scrapes into the formal delineation of a novel. However, this was unquestionably the book that fired the imaginations of not just the beat generation but subsequent generations of youthful travellers who drove, hitched, bummed or walked across America in search of epiphany, or meaning, or simply fun. It is in many way the very template of a road novel - picaresque, ambitious, episodic and (also a central part of many road novels) rebellious or countercultural in theme.

My second choice suffers the weakness of the first - in that it verges on non-fiction - but Zen and the Art of Motorcycle Maintenance, by Robert M Pirsig, in its description of a father and son's travels through American on a Harley-Davidson motorbike is in every other way the perfect example of a road novel. Although philosophically challenging - it is in some ways a philosophy primer on wheels - the ruminations on the meaning and experience of travel, the centrality of the parallels between the inner and outer journey and the grand geographical sweep of the narrative, make it central to the road canon.

My final choice violates several of my self-imposed rules for the road genre. However, it has to be included because it would have had cars and roads if they had been invented, and because it is simply so damn enjoyable. Larry McMurtry's Lonesome Dove is the story of a bunch of cowboys travelling from a ranch in Texas to discover the wilderness of Montana. The deft characterisations, the skill in the unfurling of a lengthy and complicated narrative, the acutely recorded trials of journeying combined with the sheer sense of adventure make it my favourite-ever road book - despite the fact that there's barely a road in sight.

• Tim Lott's memoir, The Special Relationship: A Love Story, will be published by Simon and Schuster in 2010

The last booklet in our series is essentially about journeys. Some are journeys of leisure, the kind of expeditions into faraway lands and adventures in exotic locations we loved reading about as children: tall tales about treasure islands, daredevil pilots and colonial exploits